Basal midbrain modulation of tonic immobility in the toad Bufo paracnemis.

Tonic immobility (TI) is considered to be a final stage in a sequence of defensive responses occurring in the prey/predator encounter. It is known that the basal midbrain of toads is involved in the organization of defensive behavior and analgesia. This study investigated the effect of electrolytic or neurotoxic lesions of two mesencephalic regions [tegmentum (TEG) and interpeduncular nucleus (IPN)] on the latency and duration of TI (induced by postural inversion and by movement restriction) and on the latency of the motor response to a nociceptive stimulus (hot plate) in toads. Electrolytic lesions of TEG and IPN promoted an increase in the duration of TI episodes. Neurotoxic lesion of these two regions also caused an increase in the duration of TI episodes. The effect was more intense in the animals with electrolytic lesion, possibly due to more extensive damage associated with this procedure or to damage of passage fibers. The results suggest that lesions of the midbrain TEG liberate basic circuits placed caudally and are involved in the organization of the TI response. It remains to be determined if the IPN exerts its effect directly on the caudal levels or by acting via the mesencephalic TEG. Lesions do not interfere with the latency of the motor response to a thermal noxious stimulus, indicating that the lesioned regions do not affect the reflexive response and are not essential for the perception of the noxious stimulus.